1. Field of the Invention
The present invention relates to an optical pickup actuator, and more particularly to an optical pickup actuator suitable for a compact/portable information equipments, such as a notebook computor.
2. Description of the Related Art
Recently, a rapid development of an optical disc has brought a variety of optical pickups for recording information on the optical disc or reproducing information therefrom. The optical pickup is provided with an actuator for tracking control and focusing control, wherein the tracking control is to control a light spot condensed by an objective lens to follow a center of a signal track on the optical disc, and the focusing control is to control the light spot to be focused on a signal track surface. This actuator is driven by a Lorenz force generated in accordance with Fleming""s left-hand law by placing a coil within a magnetic field space between a magnet and a magnetic substance.
The optical pickup is tending to be made thinner to keep pace with compact/portable information equipments, such as a notebook computer. The optical pickup actuator can be classified into two types according to a position of the objective lens on a bobbin, i.e. a lens-centering type as shown in FIG. 1 and a lens-protruding type as shown in FIG. 4.
Referring to FIG. 1, a conventional lens-centering type actuator is divide into a moving part including an objective lens 2, a bobbin 4, tracking coils 10, focusing coils 12 and wire springs 14, and a fixed part including permanent magnets 6 and a yoke 8. In the moving portion, the objective lens 2 serves to condense an incident light beam from a light source on an optical disc. The objective lens 2 is fitted into an annular hole formed in a center portion of the bobbin 4. The focusing coils 12 are wound around the whole side surfaces of the bobbin 4, and the tracking coils 10 are adhered to the wound surfaces of the focusing coils 12. The wire springs 14 are connected between printed circuit boards (not shown) disposed at centers of left/right side surfaces of the bobbin 4 and a frame to support elastically the moving part and to supply a current signal from the frame to the tracking coils 10 and the focusing coils 12. In the fixed part, the permanent magnets 6 are adhered to the yoke 8 while confronting the tracking coils 10 and the focusing coils 12 to generate a magnetic flux interlinking with the tracking coils 10 and the focusing coils 12. The yoke 8 is composed of a metallic magnetic substance, outer side portions of which the permanent magnets are adhered to and opposite inner side portions of which are fitted into rectangular holes in the bobbin 4.
With this lens-centering type actuator, as shown in FIG. 2a, the direction of a focusing drive force is determined by the direction of the magnetic flux generated by the permanent magnets 6 and the direction of electric current applied to the focusing coils 12. For example, when the direction of the magnetic flux is a direction of x-axis and the direction of electric current flowing in the focusing coils 12 within a magnetic field space is a direction of z-axis (a direction coming from a land surface), the driving force acts in a direction of y-axis in accordance with Fleming""s left-hand law. Similarly, when the direction of the magnetic flux is a direction ofxe2x80x94x-axis and the direction of electric current is a direction of z-axis, the driving force acts in a direction ofxe2x80x94y-axis. This force acting in the vertical direction drives the objective lens 2 in a direction perpendicular to a recording surface of the optical disc.
As shown in FIG. 2b, the direction of tracking drive force is determined by the direction of the magnetic flux generated by the permanent magnets 6 and the direction of electric current applied to the tracking coils 10. For example, when the direction of the magnetic flux is a direction of z-axis and the direction of electric current is a direction of y-axis, the driving force acts in a direction of x-axis. Similarly, when the direction of the magnetic flux is a direction of z-axis and the direction of electric current is a direction ofxe2x80x94y-axis, the driving force acts in a direction ofxe2x80x94x-axis. This force acting in the horizontal direction drives the objective lens 2 in a direction horizontal to the recording surface of the optical disc.
However, there is a limitation in making the actuator thin enough in the lens-centering type actuator because a magnetic circuit is constructed on an optical path of the incident light from the light source. In fact, the lens-centering type actuator is accompanied with structural difficulties in constructing the magnetic circuit on the optical path, thus a height HLCA from the objective lens 2 to a 45xc2x0 reflecting mirror 16 becomes high as shown in FIG. 3.
To solve this problem, a lens-protruding type actuator has been proposed which is constructed as shown in FIGS. 4 and 5. Referring to FIGS. 4 and 5, the lens-protruding type actuator is divided into a moving part including an objective lens 22, a bobbin 24, tracking coils 30, focusing coils 32 and wire springs 34, and a fixed part including permanent magnets 26 and a yoke 28. In the moving part, the bobbin 24 protrudes a semicircle on its one side, and the objective lens 22 is clamped in a center portion of the protruded side of the bobbin 24. The tracking coils 30 and the focusing coils 32 are disposed within the bobbin 24 while confronting the permanent magnets 26. The wire springs 34 are connected between printed circuit boards (not shown) positioned at centers of left/right side surfaces of the bobbin 24 and a frame. In the fixed part, the permanent magnets 26 are adhered to the side surfaces of the yoke 28 while confronting the tracking coils 30 and the focusing coils 32. Both sides of the yoke 28 are fitted into rectangular holes of the bobbin 24 interposing the tracking coils 30 and the focusing coils 32 therebetween.
Since the objective lens 22 protrudes toward a light source, a magnetic circuit of this lens protruding type actuator can be arranged inside the bobbin 24 as to be positioned outside the optical path. Thus, the height HLPA from the objective lens 22 to a 45xc2x0 reflecting mirror 36 becomes low as shown in FIG. 5.
With this lens-protruding type actuator, the magnetic circuit structure is disposed in a center portion of the bobbin 24 far from the objective lens 22 in order to avoid the optical path, and the moving part has an asymmetric structure with respect to the objective lens 22. This asymmetric structure of the moving part in the lens-protruding type actuator causes an inconsistency that a center of mass Cmass does not converge into both centers of tracking/focusing movements TC, FC as shown in FIG. 6. As a result of this, there is a problem in the conventional lens-protruding type actuator that a vibration mode of the wire springs 34 is found in a driving frequency band making the actuator easily excited.
The moving part of the lens-protruding type actuator is vibrated in a rotational vibration mode due to the inconsistency of the center of gravity with the centers of driving movements of the moving part as shown in FIGS. 7a and 7b. FIG. 7a depicts a rolling mode in which the moving part rotates with an angle with respect to a tangential direction (x-axis) of the optical disc. FIG. 7b depicts a pitching mode in which the moving part rotates with an angle with respect to a radial direction (y-axis) of the optical disc. FIG. 7c depicts a yawing mode in which the moving part rotates with an angle with respect to a direction of an optical axis (z-axis) perpendicular to the optical disc. When the moving part is driven along the tracking or focusing direction in these rotational vibration modes, there is a slant movement of the moving part to cause a phase change of the objective lens 22. That is, the lens-protruding type actuator has a problem in that control stability is low. Particularly, the control stability is low in the pitching mode as compared with in any other modes because the objective lens 22 is positioned in the front area of the asymmetrical moving part. In detail, in this lens-protruding type actuator, the focusing coils in the magnetic circuit structure which generates the driving force of the actuactor are wound around one side of the yoke 28 in a rear portion of the bobbin 24, and thus comprise a main focusing coil 32a positioned within an effective magnetic field space and a sub focusing coil 32b positioned in the rear portion of the bobbin 24 as seen from FIG. 8a. A magnetic flux 26a coming from the permanent magnet 26 interlinks with the main focusing coil 32a within the magnetic field space to generate a driving force along a focusing direction while a part 26b of the magnetic flux leaks backward the bobbin 24 interlinking with the sub focusing coil 32b. Consequently, the driving force generated from the side of the main focusing coil 32a along the focusing direction and an incidental force generated from the side of the sub focusing coil 32b give rise to a moment to excite the pitching mode of the moving part. When the moving part is excited in the focusing direction as shown in FIG. 8b, the objective lens 22 in the moving part suffers a great phase change in a resonance frequency band of a characteristic frequency response in which the pitching mode occurs. At this time of a phase change in pitching mode, the gain is also changed. Quantity of this phase change can be reduced by changing the position of the center of mass of the moving part. In other words, if a ratio of the magnetic flux influencing the main focusing coil 32a and the leakage magnetic flux influencing the sub focusing coil 32b is adjusted by changing the position of the center of mass of the moving part, the phase change of the objective lens 22 in the pitching mode can be reduced. In this way, a condition where no phase change occur can be realized by adjusting the position of the center of mass of the moving part and thus changing the moment of the moving part. For the purpose of this, the conventional lens-protruding type actuator is provided with a mass balancer for shifting the position of the center of mass toward the centers of tracking/focusing movements. However, there is a limitation of a space for disposing the mass balancer due to the current trend, i.e. thin actuactor, and also may be aroused unbalance of center of mass to grow heavier.
Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide an optical pickup actuator capable of regulating quantity of a leakage magnetic flux to minimize a phase change of an objective lens.
To achieve this object, there is provided an optical pickup actuator comprising:
a moving part having a asymmetric structure with respect to a objective lens; and
a fixed part including permanent magnets and a yoke;
wherein a leakage magnetic flux generated from a magnetic circuit constructed by a permanent magnets, a yoke and tracing/focusing coils, interlinking with a sub coil portion of the focusing coils are controlled by a magnetic flux-controlling member.
According to the present invention, a phase change of the objective lens in a rotational vibration mode, particularly, in a pitching mode can be minimized by changing a moment of force generated from coils outside of an effective magnetic field space and influencing the movable part.
The magnetic flux-controlling member is preferably a cap member for covering an opening formed at an upper portion of the yoke. A length of the cap member is determined in accordance with distances between a center of mass and centers of driving movements of the movable part. Preferably, the cap member is formed with holes having a predetermined size.
Alternatively, the magnetic flux-controlling member is the yoke having an opening formed on its sidewall confronting the sub coil portion of the focusing coils.
It is preferred that the sub coil portion of the focusing coils is integrated to a main focusing coil while having a plurality of inclined portions meeting with both end of the main focusing coil at a predetermined tilt angle.